Methods and system for performing handover in a wireless communication system

ABSTRACT

A method and system for performing handover in a third generation (3G) long term evolution (LTE) system are disclosed. A source evolved Node-B (eNode-B) makes a handover decision based on measurements and sends a handover request to a target eNode-B. The target eNode-B sends a handover response to the source eNode-B indicating that a handover should commence. The source eNode-B then sends a handover command to a wireless transmit/receive unit (WTRU). The handover command includes at least one of reconfiguration information, information regarding timing adjustment, relative timing difference between the source eNode-B and the target eNode-B, information regarding an initial scheduling procedure at the target eNode-B, and measurement information for the target eNode-B. The WTRU then accesses the target eNode-B and exchanges layer 1/2 signaling to perform downlink synchronization, timing adjustment, and uplink and downlink resource assignment based on information included in the handover command.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application No.60/815,023 filed Jun. 20, 2006, which is incorporated by reference as iffully set forth.

FIELD OF THE INVENTION

The present invention is related to wireless communication systems. Moreparticularly, the present invention is related to a method and systemfor performing handover in a long term evolution (LTE) system.

BACKGROUND

LTE for the fourth generation (4G) system is now being considered todevelop a new radio interface and radio network architecture thatprovides a high data rate, low latency, packet optimization, andimproved system capacity and coverage. For an LTE system, instead ofusing code division multiple access (CDMA), which is currently beingused in a 3G system, orthogonal frequency division multiple access(OFDMA) and frequency division multiple access (FDMA) are proposed to beused in downlink and uplink transmissions, respectively. By changing inmany aspects in the LTE system, intra-LTE handover procedures andrelated operations need to be re-considered.

The user equipment (UE) mobility management in an LTE_ACTIVE modehandles all necessary steps for seamless handover in the LTE system,such as making an intra-LTE handover decision on a source network side,(i.e., control and evaluation of UE and evolved Node-B (eNode-B)measurements taking into account UE-specific area restrictions),preparing radio resources on a target network side, commanding the UE tointerface with new radio resources, releasing radio resources on thesource network side, and the like. The UE mobility management mechanismalso handles the transfer of context data between involved nodes, andthe update of node relations on a control plane (C-plane) and a userplane (U-plane).

FIG. 1 is a signaling diagram of a handover process 100 currentlyproposed for the LTE system. A UE 152 and a source eNode-B 154 performmeasurements and exchange measurement reports (step 102). The sourceeNode-B 154 makes a handover decision based on the measurement reports(step 104). The source eNode-B 154 then sends a handover request to atarget eNode-B 156 (step 106). The handover decision and subsequentprocedures before handover completion are performed without involving amobility management entity/user plane entity (MME/UPE) 158, (i.e.,handover preparation messages are directly exchanged between the sourceeNode-B 154 and the target eNode-B 156).

The target eNode-B 156 performs an admission control for the UE 152(step 108). If the target eNode-B 156 can accept the UE 152, the targeteNode-B 156 sends a handover response to the source eNode-B 154 (step110). The source eNode-B 154 sends a handover command to the UE 152(step 112). For seamless handover, a U-plane tunnel is establishedbetween the source eNode-B 154 and the target eNode-B 156.

The UE 152 and the target eNode-B 156 then exchange layer 1 and 2(L1/L2) signaling (step 114). During handover execution, user data maybe forwarded from the source eNode-B 154 to the target eNode-B 156. Theforwarding may take place in a service dependent and implementationspecific way. Forwarding of user data from the source eNode-B 154 to thetarget eNode-B 156 should take place as long as packets are received atthe source eNode-B 154 from the UPE 158.

After a connection to the target eNode-B 156 is established, the UE 152sends a handover complete message to the target eNode-B 156 (step 116).The target eNode-B 156 sends a handover complete message to the MME/UPE158 (step 118). The MME/UPE 158 then sends a handover completeacknowledgement (ACK) to the target eNode-B 156 (step 120). After theMME/UPE 158 is informed by the target eNode-B 156 that the UE 152 hasgained an access at the target eNode-B 156 by the handover completemessage, the U-plane path is switched by the MME/UPE 158 from the sourceeNode-B 154 to the target eNode-B 156.

The release of the radio resources at the source eNode-B 154 istriggered by a release resource message sent by the target eNode-B 156(step 122). After receiving the release resource message from the targeteNode-B 156, the source eNode-B 154 releases the radio resources for theUE 152 (step 124). The UE 152 performs a location update with theMME/UPE 158 (step 126).

The above intra-LTE handover procedure 100 does not provide detailsregarding the handover command, (such as configurations of the UE 152based on the target eNode-B's requirement), and details regarding UEoperation after the UE receives the handover command, (such as datatransmission between the source eNode-B 154 and the UE 152 and radiolink control (RLC) and hybrid automatic repeat request (HIARQ) reset andpacket data convergence protocol (PDCP) sequence number (SN) gapidentification by the UE 152). The above intra-LTE handover procedure100 also does not provide details regarding UE timing adjustment forsynchronous and asynchronous eNode-Bs and details for efficient targeteNode-B scheduling of resources for UE transmission.

SUMMARY

The present invention is related to a method and system for performinghandover in an LTE system. A source eNode-B makes a handover decisionbased on measurements, and sends a handover request to a target eNode-B.The target eNode-B sends a handover response to the source eNode-Bindicating that a handover should commence. The source eNode-B thensends a handover command to a wireless transmit/receive unit (WTRU). Thehandover command includes at least one of reconfiguration information,information regarding timing adjustment, relative timing differencebetween the source eNode-B and the target eNode-B, information regardingan initial scheduling process at the target eNode-B, and measurementinformation for the target eNode-B. The WTRU then accesses the targeteNode-B and exchanges layer 1/2 signaling to perform downlinksynchronization, timing adjustment, and uplink and downlink resourceassignment based on information included in the handover command.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawingswherein:

FIG. 1 is a signaling diagram of a handover process currently proposedfor the LTE system; and

FIG. 2 is a signaling diagram of an intra-LTE handover process inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When referred to hereafter, the terminology “WTRU” includes but is notlimited to a UE, a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), acomputer, or any other type of user device capable of operating in awireless environment. When referred to hereafter, the terminology“eNode-B” includes but is not limited to a base station, Node-B, a sitecontroller, an access point (AP), or any other type of interfacingdevice capable of operating in a wireless environment.

The present invention provides detailed procedures for signaling andoperations at a WTRU and source and target eNode-Bs during intra-LTEhandover both for successful handover and handover failure cases. In asuccessful handover case, new information elements (IEs) are added inboth the handover command message and the handover complete message. Ina handover failure case, new signaling messages are exchanged between asource eNode-B and a target eNode-B.

FIG. 2 is a signaling diagram of an intra-LTE handover process 200 inaccordance with the present invention. A WTRU 252 and a source eNode-B254 each perform at least one measurement, and the WTRU 252 sends ameasurement report to the source eNode-B 254 (step 202). The sourceeNode-B 254 makes a handover decision based on the measurement reportand the result of its own measurement (step 204). The source eNode-B 254then sends a handover request to a target eNode-B 256 (step 206). Thetarget eNode-B 256 performs an admission control for the WTRU 252 (step208). If the target eNode-B 256 can accept the WTRU 252, the targeteNode-B 256 sends a handover response to the source eNode-B 254indicating that a handover should commence (step 210). The sourceeNode-B 254 then sends a handover command to the WTRU 252 (step 212).

The handover command should include at least one of reconfigurationinformation for radio resource control (RRC), radio link control (RLC),medium access control (MAC) and physical (PHY) layer, informationregarding timing adjustment when handing over from the source eNode-B254 to the target eNode-B 256, (i.e., whether the WTRU 252 shouldperform timing adjustment autonomously or using a random access channel(RACH) procedure, if a RACH is to be used, whether random or dedicatedaccess signature will be used, or the like), relative timing differencebetween eNode-Bs (or cells) for autonomous timing adjustment,information regarding initial radio resource scheduling procedure at thetarget eNode-B 256, measurement information for the target eNode-B 256,and the like. The information regarding the initial scheduling procedureat the target eNode-B 256 indicates whether a RACH access procedureshould be used for a resource assignment request or the target eNode-B256 may schedule resources for the WTRU 252 without receiving anexplicit resource assignment request from the WTRU 252. Alternatively,the measurement and other configuration information may be sent to theWTRU 252 by the target eNode-B 256 after receiving a handover completemessage from the WTRU 252 at step 226.

For a seamless handover, a U-plane tunnel is established between thesource eNode-B 254 and the target eNode-B 256. After sending thehandover command, the source eNode-B 254 may forward the user data tothe target eNode-B 256. The forwarding may take place in a servicedependent and implementation specific way.

After receiving the handover command from the source eNode-B 254, theWTRU 252 may continue to transmit and receive data to and from thesource eNode-B 254. The data transmission process depends on whethersynchronized handover or non-synchronized handover is used.

When a synchronized handover procedure is used, (i.e., the sourceeNode-B 254 and the target eNode-B 256 are synchronized or the relativetiming difference is known to the WTRU 252), the source eNode-B 254 andthe WTRU 252 may continue to transmit and receive data after receivingthe handover command until a certain handover time (t_(HO)) which issignaled via the handover command. The transmitted data after receivingthe handover command is preferably limited to incomplete service dataunits (SDUs), (i.e., RLC protocol data unit (PDU)), transmitted beforethe handover command was sent. An RLC control message is sent to theWTRU 252 to indicate a sequence number (SN) of a successfully receivedSDU(s) and an SDU gap. The SN may be a PDCP SN, or other types of SN. AnSN common to the successfully received SDU(s) and unsuccessfullyreceived SDU(s) may be included in the RLC control message.

When a non-synchronized handover procedure is used, (i.e., the sourceeNode-B 254 and the target eNode-B 256 are not synchronized or therelative timing difference is not known to the WTRU 252), the sourceeNode-B 254 stops transmission as soon as the source eNode-B 254 sendsthe handover command to the WTRU 252. The WTRU 252 also stopstransmission of the data packets to the source eNode-B 254 as soon asthe WTRU 252 receives the handover command. Alternatively, the sourceeNode-B 254 may continue transmission of data packets until the WTRU 252switches to the target eNode-B 254.

After receiving the handover command, the WTRU 252 accesses the targeteNode-B 256 and exchange layer 1/2 (L1/L2) signaling with the targeteNode-B 256 to perform downlink synchronization, timing adjustment,(i.e., uplink synchronization), and uplink and downlink resourceassignment based on information included in the handover command.

For timing adjustment, (i.e., uplink synchronization), the WTRU 252implements one of two options. Preferably, the network decides whichoption to be used.

In accordance with a first option, the WTRU 252 autonomously performsthe timing adjustment based on relative timing difference between thesource eNode-B 254 (or cell) and the target eNode-B 256 (or cells) (step214 a). The relative timing difference information is preferablyincluded in the handover command.

In accordance with a second option, a conventional RACH access procedureis used for the timing adjustment (step 214 b). The WTRU sends a RACHpreamble to the target eNode-B and the target eNode-B calculates timingoffset based on the transmitted RACH preamble and sends the timingoffset information to the WTRU for uplink synchronization.

A plurality of RACH preamble signatures with different orthogonality anddifferent priority may be used, and among the plurality of RACH preamblesignatures, a RACH preamble signature with higher orthogonality, higherpriority and/or higher power may be used for the handover purpose.

A particular (dedicated) RACH preamble signature may be reserved for thehandover purpose to indicate that the sender is a handover WTRU, (i.e.,a WTRU undergoing a handover process). This dedicated RACH preamblesignature is indicated in the handover command. After receiving thereserved RACH preamble signature, the target eNode-B 256 recognizes thatthe sender is a handover WTRU and may provide a priority to the handoverWTRU. This can avoid the random access process which causes a longinterruption time during handover. Alternatively, a RACH messagefollowing the RACH preamble may explicitly indicate that the sender is ahandover WTRU. A handover WTRU is preferably given a higher priority toaccess an eNode-B (cell) than a non-handover WTRU due to statetransition. The RACH procedure using the reserved RACH preamblesignature may be used in either synchronized or non-synchronized eNode-B(or cell) handover. A physical radio resource allocation for sending thereserved RACH preamble signature to the target eNode-B 256 may also beincluded in the handover command to reduce a delay for the randomaccess.

The random access procedure may be used for different purposes. Therandom access procedure may be used to initiate communication between aWTRU and a network which requires a state transit from an LTE_idle stateto an LTE_active state. The random access procedure may be used fortiming adjustment during handover and then for an access request to thenew cell. When the random access procedure is used during handover, thedelay caused by the random access procedure should be minimized.Therefore, there should be differences, (e.g., giving a priority to ahandover WTRU), between the random access to the target eNode-B (cell)during handover and the random access to the source eNode-B (cell) in anon-handover situation because of state transition from an LTE-Idlestate to an LTE-Active state in the non-handover case.

After receiving the RACH preamble signature from the WTRU, the targeteNode B estimates the timing adjustment value and sends this value backto the WTRU (step 216).

After performing timing adjustment, (either autonomously or via a RACHpreamble transmission), the WTRU 202 may send a radio resourceassignment request to the target eNode-B 256 (step 218). The request ispreferably sent via a RACH message following the RACH preamble. Thetarget eNode-B 256 then schedules downlink and uplink resources for theWTRU 252 (step 220). Alternatively, the target eNode-B 256 may scheduleresources for the WTRU 252 without receiving an explicit request fromthe WTRU 252. The resource scheduling may take place any time after thetarget eNode-B 256 admits the WTRU at step 208. For example, for thesynchronized handover procedure, the target eNode-B 256 may schedule theuplink and downlink resources after some pre-defined time (earlier thanthe expected time for eNode-B switching).

The target eNode-B 256 sends an uplink resource assignment to the WTRU252 (step 222). This uplink resource is used for sending a handovercomplete message at step 226, not for data transmission. The WTRU 252preferably resets RLC and HARQ parameters after receiving the uplinkresource assignment from the target eNode-B 256 (step 224).Alternatively, the WTRU 252 may reset the RLC and HARQ parameters afterreceiving and processing the handover command at step 212. Theseparameters related to transmission to the target eNode-B 256 (or cell)are included in the handover command.

The WTRU 252 sends a handover complete message to the target eNode-B 256(step 226). The WTRU 252 preferably includes a starting uplink PDCP SNto be transmitted in the handover complete message. Optionally, the WTRU252 may send an RLC control message to the target eNode-B 256 after thehandover complete message to indicate the successfully transmitted SDUsand an SDU gap.

The target eNode-B 256 sends uplink and downlink resource schedulinginformation for data transmission and an RRC message to the WTRU (step228). The RRC message includes at least one of radio access bearer (RAB)reconfiguration information, a starting PDCP SN in the downlink, an RLCcontrol message, and measurement related information. Some or all of theabove information may optionally be sent as part of the handover commandor the first packet from the target eNode-B 256.

The target eNode-B 256 sends a handover complete message to the MME/UPE258 to inform that the WTRU 252 has gained an access at the targeteNode-B 256 (step 230). The MME/UPE 258 then sends a handover completeacknowledgement (ACK) to the target eNode-B 256 and switches the U-planedata path from the source eNode-B 254 to the target eNode-B 256 (step232). A release of the radio resources at the source eNode-B 254 istriggered by a release resource message sent by the target eNode-B 256(step 234). After receiving the message from the target eNode-B 256, thesource eNode-B 254 releases the radio resources for the WTRU 252 (step236).

A handover failure case is explained hereinafter by referring to FIG. 2.When the WTRU 252 is not able to handover successfully, the WTRU 252 mayresort to a radio link (RL) failure or a cell reselection procedure. Ifthe handover command fails at step 212, the source eNode-B 254 informsthe target eNode-B 256 of such a failure. The target eNode-B 256schedules any uplink and downlink resources to the WTRU 252 after step208. When performing cell reselection in a handover failure case, theWTRU 252 may first try to access the originally connected cell withinthe source eNode-B 254. If this fails, the WTRU 252 may try to accessother cells within the source eNode-B. If this also fails, then the WTRU252 may try to access to other cells not included in the source eNode-Bbased on the measurement result.

The source eNode-B 254 maintains a timer to time out if the handovercomplete message is not received after a predetermined time after thehandover command failure. The source eNode-B 254 may reset RRC context,PDCP context, RLC and HARQ parameters related to the WTRU 252 if thehandover failure timer expires. The source eNode-B then releases theradio resources for the WTRU 252.

When cell reselection is performed by the WTRU 252, the source cell oreNode-B identity (ID) is sent by the WTRU 252 to any eNode-B as part ofthe LTE-radio network temporary identity (RNTI) information for thedetection if the WTRU 252 accesses the original cell or any other cells.At the source eNode-B, the source eNode-B's MAC layer informs its RRClayer of the handover failure if the MAC layer detects failedtransmission of handover command.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Themethods or flow charts provided in the present invention may beimplemented in a computer program, software, or firmware tangiblyembodied in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module.

1. A method for performing handover in a wireless communication system,the method comprising: a wireless transmit/receive unit (WTRU) and asource evolved Node-B (eNode-B) performing measurements; the sourceeNode-B making a handover decision based on the measurements; the sourceeNode-B sending a handover request to a target eNode-B; the targeteNode-B sending a handover response to the source eNode-B indicatingthat a handover should commence; and the source eNode-B sending ahandover command to the WTRU, the handover command including at leastone of reconfiguration information, information regarding timingadjustment, relative timing difference between the source eNode-B andthe target eNode-B; information regarding an initial radio resourcescheduling procedure at the target eNode-B, and measurement informationfor the target eNode-B.
 2. The method of claim 1 wherein thereconfiguration information is for at least one of a radio resourcecontrol (RRC) layer, a radio link control (RLC) layer, a medium accesscontrol (MAC) layer and a physical layer.
 3. The method of claim 1wherein the handover command indicates that the target eNode-B schedulesresource for the WTRU based on a random access channel (RACH) accessprocedure.
 4. The method of claim 1 wherein the handover commandindicates that the target eNode-B schedules resource for the WTRUwithout receiving an explicit resource assignment request from the WTRU.5. The method of claim 1 further comprising: the source eNode-Bforwarding user data to the target eNode-B.
 6. The method of claim 5wherein the forwarding of the user data is performed in a servicedependent and implementation specific way.
 7. The method of claim 1wherein the WTRU and the source eNode-B continue to transmit and receivedata after the WTRU receives the handover command.
 8. The method ofclaim 7 wherein the WTRU and the source eNode-B continue to transmit andreceive the data until a handover time that is signaled via the handovercommand.
 9. The method of claim 7 wherein the data transmitted is anincomplete service data unit (SDU).
 10. The method of claim 9 whereinthe source eNode-B sends a radio link control (RLC) message to the WTRUincluding a sequence number (SN) to indicate a successfully received SDUand an unsuccessfully received SDU.
 11. The method of claim 10 whereinthe SN is a packet data convergence protocol (PDCP) SN or a common SN.12. The method of claim 1 wherein the source eNode-B stops transmissionof data to the WTRU as soon as the source eNode-B sends the handovercommand to the WTRU, and the WTRU stops transmission of data to thesource eNode-B as soon as the WTRU receives the handover command. 13.The method of claim 1 wherein the source eNode-B continues transmissionof data until the WTRU switches to the target eNode-B.
 14. The method ofclaim 1 further comprising: the WTRU performing timing adjustment withthe target eNode-B.
 15. The method of claim 14 wherein the WTRUautonomously performs the timing adjustment based on relative timingdifference between the source eNode-B and the target eNode-B.
 16. Themethod of claim 15 wherein the relative timing difference information isincluded in the handover command.
 17. The method of claim 14 wherein theWTRU uses a random access channel (RACH) access procedure for the timingadjustment.
 18. The method of claim 17 wherein a plurality of RACHpreamble signatures with different orthogonality and different priorityare used, and among the plurality of RACH preamble signatures, a RACHpreamble signature with higher orthogonality, higher priority and higherpower is used for handover purpose.
 19. The method of claim 18 wherein aparticular RACH preamble signature is reserved for the handover purpose.20. The method of claim 19 wherein the reserved RACH preamble signatureis indicated in the handover command.
 21. The method of claim 14 furthercomprising: the target eNode-B assigning an uplink resource fortransmission of a handover complete message for the WTRU.
 22. The methodof claim 21 wherein the target eNode-B schedules the uplink resourcebased on a resource assignment request from the WTRU.
 23. The method ofclaim 22 wherein the resource assignment request is sent via a randomaccess channel (RACH).
 24. The method of claim 21 wherein the targeteNode-B schedules the uplink resource without receiving a request fromthe WTRU.
 25. The method of claim 21 further comprising: the WTRUresetting radio link control (RLC) and hybrid automatic repeat request(HARQ) after receiving the uplink resource from the target eNode-B. 26.The method of claim 1 further comprising: the WTRU resetting radio linkcontrol (RLC) and hybrid automatic repeat request (HARQ) after receivingthe handover command.
 27. The method of claim 26 further comprising: theWTRU sending a handover complete message to the target eNode-B, thehandover complete message including an uplink packet data convergenceprotocol (PDCP) sequence number (SN) to be transmitted.
 28. The methodof claim 27 further comprising: the WTRU sending an RLC control messageto the target eNode-B after the handover complete message to indicate asuccessfully transmitted service data unit (SDU) and an SDU gap.
 29. Themethod of claim 27 further comprising: the target eNode-B sending uplinkand downlink scheduling information for data transmission and radioresource control (RRC) message to the WTRU, the RRC message including atleast one of radio access bearer (RAB) reconfiguration information, astarting PDCP SN start in a downlink, an RLC control message, andmeasurement related information.
 30. The method of claim 1 furthercomprising: the WTRU performing a radio link (RL) failure procedure whenthe handover command is not successfully delivered.
 31. The method ofclaim 1 wherein the source eNode-B maintains a timer to time out if ahandover complete message is not received until a predetermined timeafter the handover command is not successfully delivered.
 32. The methodof claim 31 wherein the source eNode-B resets radio resource control(RRC) context, packet data convergence protocol (PDCP) context, radiolink control (RLC) and hybrid automatic repeat request (HARQ) parametersrelated to the WTRU if the timer expires.
 33. The method of claim 1further comprising: the WTRU performing a cell reselection procedurewhen the handover command is not successfully delivered.
 34. The methodof claim 33 wherein the WTRU first tries to access an originallyconnected cell in the source eNode-B.
 35. The method of claim 34 whereinthe WTRU tries to access another cell in the source eNode-B if the WTRUfails to access the originally connected cell.
 36. The method of claim35 wherein the WTRU tries to access another cell not included in thesource eNode-B if the WTRU fails to access said another cell in thesource eNode-B.
 37. The method of claim 33 wherein the WTRU sends asource eNode-B identity (ID) to the target eNode-B during cellreselection.
 38. A wireless communication system for performinghandover, the system comprising: a wireless transmit/receive unit (WTRU)configured to perform measurement and send a measurement repot; a targetevolved Node-B (eNode-B); and a source eNode-B configured make ahandover decision based on the measurement report, send a handoverrequest to the target eNode-B, and send a handover command to the WTRUafter receiving a handover response from the target eNode-B indicatingthat a handover should commence, wherein the handover command includesat least one of reconfiguration information, information regardingtiming adjustment, relative timing difference between the source eNode-Band the target eNode-B; information regarding an initial radio resourcescheduling procedure at the target eNode-B, and measurement informationfor the target eNode-B.
 39. The system of claim 38 wherein thereconfiguration information is for at least one of a radio resourcecontrol (RRC) layer, a radio link control (RLC) layer, a medium accesscontrol (MAC) layer and a physical layer.
 40. The system of claim 38wherein the handover command indicates that the target eNode-B schedulesresource for the WTRU using a random access channel (RACH) accessprocedure.
 41. The system of claim 38 wherein the handover commandindicates that the target eNode-B schedules resource for the WTRUwithout receiving an explicit resource assignment request from the WTRU.42. The system of claim 38 wherein the source eNode-B is configured toforward user data to the target eNode-B after sending the handovercommand to the WTRU.
 43. The system of claim 42 wherein the forwardingof the user data is performed in a service dependent and implementationspecific way.
 44. The system of claim 38 wherein the WTRU and the sourceeNode-B continue to transmit and receive data after the WTRU receivesthe handover command.
 45. The system of claim 44 wherein the WTRU andthe source eNode-B continue to transmit and receive the data until ahandover time that is signaled via the handover command.
 46. The systemof claim 44 wherein the data transmitted is an incomplete service dataunit (SDU).
 47. The system of claim 46 wherein the source eNode-B sendsa radio link control (RLC) message to the WTRU including a sequencenumber (SN) to indicate a successfully received SDU and anunsuccessfully received SDU.
 48. The system of claim 47 wherein the SNis a packet data convergence protocol (PDCP) SN or a common SN.
 49. Thesystem of claim 38 wherein the source eNode-B stops transmission of datato the WTRU as soon as the source eNode-B sends the handover command tothe WTRU, and the WTRU stops transmission of data to the source eNode-Bas soon as the WTRU receives the handover command.
 50. The system ofclaim 38 wherein the source eNode-B continues transmission of data untilthe WTRU switches to the target eNode-B.
 51. The system of claim 38wherein the WTRU is configured to perform timing adjustment with thetarget eNode-B.
 52. The system of claim 51 wherein the WTRU isconfigured to autonomously perform the timing adjustment based onrelative timing difference between the source eNode-B and the targeteNode-B.
 53. The system of claim 52 wherein the relative timingdifference information is included in the handover command.
 54. Thesystem of claim 51 wherein the WTRU is configured to use a random accesschannel (RACH) access procedure for the timing adjustment.
 55. Thesystem of claim 54 wherein plurality of RACH preamble signatures withdifferent orthogonality and different priority are used, and among theplurality of RACH preamble signatures, a RACH preamble signature withhigher orthogonality, higher priority and higher power is used forhandover purpose.
 56. The system of claim 55 wherein a particular RACHpreamble signature is reserved for the handover purpose.
 57. The systemof claim 56 wherein the reserved RACH preamble signature is indicated inthe handover command.
 58. The system of claim 51 wherein the targeteNode-B is configured to assign an uplink resource for transmission of ahandover complete message for the WTRU.
 59. The system of claim 58wherein the target eNode-B is configured to schedule the uplink resourcebased on a resource assignment request from the WTRU.
 60. The system ofclaim 59 wherein the resource assignment request is sent via a randomaccess channel (RACH).
 61. The system of claim 58 wherein the targeteNode-B is configured to schedule the uplink resource without receivinga request from the WTRU.
 62. The system of claim 58 wherein the WTRU isconfigured to reset radio link control (RLC) and hybrid automatic repeatrequest (HARQ) after receiving the uplink resource from the targeteNode-B.
 63. The system of claim 38 wherein the WTRU is configured toreset radio link control (RLC) and hybrid automatic repeat request(HARQ) after receiving the handover command.
 64. The system of claim 63wherein the WTRU is configured to send a handover complete message tothe target eNode-B, the handover complete message including an uplinkpacket data convergence protocol (PDCP) sequence number (SN) to betransmitted.
 65. The system of claim 64 wherein the WTRU is configuredto send an RLC control message to the target eNode-B after the handovercomplete message to indicate a successfully transmitted service dataunit (SDU) and an SDU gap.
 66. The system of claim 64 wherein the targeteNode-B is configured to send uplink and downlink scheduling informationfor data transmission and radio resource control (RRC) message to theWTRU, the RRC message including at least one of radio access bearer(RAB) reconfiguration information, a starting PDCP SN start in adownlink, an RLC control message, and measurement related information.67. The system of claim 38 wherein the WTRU is configured to perform aradio link (RL) failure procedure when the handover command is notsuccessfully delivered.
 68. The system of claim 38 wherein the sourceeNode-B includes a timer to time out if a handover complete message isnot received until a predetermined time after the handover command isnot successfully delivered.
 69. The system of claim 68 wherein thesource eNode-B resets radio resource control (RRC) context, packet dataconvergence protocol (PDCP) context, radio link control (RLC) and hybridautomatic repeat request (HARQ) parameters related to the WTRU if thetimer expires.
 70. The system of claim 38 wherein the WTRU is configuredto perform a cell reselection procedure when the handover command is notsuccessfully delivered.
 71. The system of claim 70 wherein the WTRUfirst tries to access an originally connected cell in the sourceeNode-B.
 72. The system of claim 71 wherein the WTRU tries to accessanother cell in the source eNode-B if the WTRU fails to access theoriginally connected cell.
 73. The system of claim 72 wherein the WTRUtries to access another cell not included in the source eNode-B if theWTRU fails to access said another cell in the source eNode-B.
 74. Thesystem of claim 70 wherein the WTRU is configured to send a sourceeNode-B identity (ID) to the target eNode-B during cell reselection. 75.An evolved Node-B (eNode-B) for performing handover in a wirelesscommunication system, the eNode-B comprising: a transceiver fortransmitting and receiving data to and from a wireless transmit/receiveunit (WTRU); a measurement unit for performing measurements on a channelfor the WTRU; and a handover controller configured to make a handoverdecision based on the measurements and send a handover command to theWTRU, the handover command including at least one of reconfigurationinformation, information regarding timing adjustment, relative timingdifference between a source eNode-B and a target eNode-B; informationregarding an initial radio resource scheduling procedure at the targeteNode-B, and measurement information for the target eNode-B.
 76. TheeNode-B of claim 75 wherein the reconfiguration information is for atleast one of a radio resource control (RRC) layer, a radio link control(RLC) layer, a medium access control (MAC) layer and a physical layer.77. The eNode-B of claim 75 wherein the handover controller controls thetransceiver such that data is transmitted and received to and from theWTRU after the WTRU receives the handover command.
 78. The eNode-B ofclaim 75 wherein the handover controller controls the transceiver suchthat data is transmitted and received to and from the WTRU until ahandover time that is signaled via the handover command.
 79. The eNode-Bof claim 78 wherein the data transmitted is an incomplete service dataunit (SDU).
 80. The eNode-B of claim 79 wherein the handover controllercontrols the transceiver such that data transmission is stopped as soonas the handover command is sent to the WTRU.
 81. A wirelesstransmit/receive unit (WTRU) for performing handover in a wirelesscommunication system, the WTRU comprising: a transceiver fortransmitting and receiving data to and from an evolved Node-B (eNode-B);a measurement unit for performing measurements; and a controller forperforming handover from a source eNode-B to a target eNode-B inaccordance with a handover command received from the source eNode-B, thehandover command including at least one of reconfiguration information,information regarding timing adjustment, relative timing differencebetween the source eNode-B and the target eNode-B; information regardingan initial radio resource scheduling procedure at the target eNode-B,and measurement information for the target eNode-B.
 82. The WTRU ofclaim 81 wherein the reconfiguration information is for at least one ofa radio resource control (RRC) layer, a radio link control (RLC) layer,a medium access control (MAC) layer and a physical layer.
 83. The WTRUof claim 81 wherein the controller controls the transceiver such thatdata is transmitted and received to and from the source eNode-B afterthe WTRU receives the handover command.
 84. The WTRU of claim 83 whereinthe data transmitted is an incomplete service data unit (SDU).
 85. TheWTRU of claim 81 wherein the controller controls the transceiver suchthat data transmission to the source eNode-B stops as soon as the WTRUreceives the handover command.
 86. The WTRU of claim 81 wherein thecontroller is configured to perform timing adjustment with the targeteNode-B.
 87. The WTRU of claim 86 wherein the controller autonomouslyperforms the timing adjustment based on relative timing differencebetween the source eNode-B and the target eNode-B.
 88. The WTRU of claim86 wherein the controller uses a random access channel (RACH) accessprocedure for the timing adjustment.
 89. The WTRU of claim 88 wherein aplurality of RACH preamble signatures with different orthogonality anddifferent priority are used, and among the plurality of RACH preamblesignatures, a RACH preamble signature with higher orthogonality, higherpriority and higher power is used for handover purpose.
 90. The WTRU ofclaim 89 wherein a particular RACH preamble signature is reserved forthe handover purpose.
 91. The WTRU of claim 90 wherein the reserved RACHpreamble signature is indicated in the handover command.
 92. The WTRU ofclaim 81 wherein the controller sends a resource assignment request tothe target eNode-B for scheduling an uplink resource.
 93. The WTRU ofclaim 92 wherein the resource assignment request is sent via a randomaccess channel (RACH).
 94. The WTRU of claim 81 wherein the controllerperforms a cell reselection procedure when the handover command is notsuccessfully received.
 95. The WTRU of claim 94 wherein the controllerfirst tries to access an originally connected cell in the sourceeNode-B.
 96. The WTRU of claim 95 wherein the controller tries to accessanother cell in the source eNode-B if the WTRU fails to access theoriginally connected cell.
 97. The WTRU of claim 96 wherein the WTRUtries to access another cell not included in the source eNode-B if theWTRU fails to access said another cell in the source eNode-B.
 98. TheWTRU of claim 94 wherein the controller sends a source eNode-B identity(ID) to the target eNode-B during cell reselection.